1. Field of the Invention
This invention is a low-loss (less than 1 dB/km at 1.30 .mu.m) single mode fiber with low dispersion (less than 5 psec/nm-km) within the wavelength range 1.25-1.385 .mu.m and having low added loss (less than 0.25 dB/km) due to cabling.
2. Disclosure of Interest
Full appreciation of the advances represented by the inventive fiber requires at least a cursory review of certain aspects of fiber design technology.
The realization of low-loss optical fibers in the early 1970's focused research on the attainment of higher bandwidth for greater information carrying capacity. Initially, graded multimode fibers were pursued, in part, because they were easier to fabricate than single mode fibers. However, workers were always aware that single mode fibers have greater inherent potential for high bandwidth, and as years passed, the search for ever higher bandwidth fibers once again focused attention on single mode fibers.
It was known that although single mode fibers display none of the inter-mode dispersion associated with multimode fibers, they do have finite pulse spreading, and hence bandwidth limitation, due, in part, to material dispersion--the dependence of index of refraction, and consequently traversal time, on wavelength. Any pulse, by Fourier definition a combination of many different wavelengths, will therefore experience broadening when traversing the fiber. However, the material dispersion phenomenon does vanish at certain wavelengths --e.g., approximately 1.28 .mu.m for fused silica, 1.35 .mu.m for heavily doped germania silica, and 1.26 .mu.m for fluorine-doped silica --and consequently these might appear at first sight to be desirable operating wavelengths, all other considerations being equal. Nevertheless, it was found that even at the material dispersion null point relatively significant pulse broadening did occur due, in part, to waveguide dispersion--the wavelength dependence of traversal time associated with purely waveguide parameters.
First principles indicate that in certain regions of the spectrum dispersive effects associated with waveguide dispersion are of opposite sign than those associated with material dispersion. Consequently, the possibility arises that fibers may be designed with a view toward canceling material dispersion against waveguide dispersion and hence yielding essentially zero dispersion at a particular wavelength (H. Tsuchiya et al, Electronics Letters, 15, 476 (1979)). Desirable wavelengths for predetermined zero dispersion include 1.55 .mu.m where the loss properties of a silica-based fiber are lowest. [In "W-type" fibers it was found that low dispersion could be obtained over a relatively broad wavelength range, (K. Okamoto et al, Electronics Letters, 15, 729 (1979)).]
In order to obtain sufficient waveguide dispersion to cancel the material dispersion at 1.55 .mu.m in typical germania doped single mode fibers, relatively small core diameters must be used, since waveguide dispersion increases in magnitude with decreasing core diameter. The use of a graded core may permit a somewhat larger core diameter, however, the effect of core diameter on splicing always remains a serious consideration which must be carefully weighed in the design of high bandwidth single mode fibers. Furthermore, even if small core single mode fibers for operation at 1.55 .mu.m would be feasible, they would be relatively useless at the present time since there is a dearth of high quality, commercially available, spectrally narrow, light sources operating at 1.55 .mu.m. This has forced the worker in the field to focus on other spectral regions where sources are available and where local minima in transmission loss occur. Such a region where commercial sources are available and where there is a local minimum in loss, occurs in the vicinity of 1.3 .mu.m, (1.25-1.385 .mu.m), stimulating interest in single mode fibers for operation in this spectral region.
A threshold consideration for operation at shorter wavelengths, such as 1.3 .mu.m, involves the need to lower the cutoff wavelength .lambda..sub.c to values close to, but below, the operating wavelength. The cutoff wavelength is that wavelength below which higher order modes may be propagated. Most desirable transmission characteristics occur when the transmission wavelength is somewhat above, but close to, cutoff. Operation at 1.5 .mu.m allows relatively high cutoff wavelengths, i.e., approximately 1.45 .mu.m. However, single mode operation at 1.3 .mu.m requires much lower cutoff wavelengths.
The cutoff wavelength is proportional to the product of the core diameter and the square root of .DELTA., where .DELTA. is the relative index difference between the core and the cladding. Hence, for low cutoff wavelengths this product must be small. However, .DELTA. itself must be relatively small in typical single-mode fibers since in high .DELTA. fibers the material dispersion, a quantity that generally increases with increasing .DELTA.'s, would be too high to allow cancellation by waveguide dispersion at 1.3 .mu.m. This is so since the waveguide dispersion at 1.3 .mu.m is large enough to cancel material dispersion in high .DELTA. fibers only if the core diameter is extremely small. It would consequently appear that low dispersion (high bandwidth) single mode fibers for operation at 1.3 .mu.m would require relatively small values of .DELTA.. However, if .DELTA. is too small, packaging losses become too high. A satisfactory design for high bandwidth low packaging loss single mode fibers for operation in the vicinity of 1.3 .mu.m has consequently eluded workers in this field.